Allow helm-projectile-ag to ignore

helm-projectile-ag now can also ignore files and directories based on
these variables:

- grep-find-ignored-files
- grep-find-ignored-directories
- projectile-globally-ignored-directories
- projectile-globally-ignored-files
Ignore files properly with helm-projectile-grep

Fix a bug where helm-projectile-grep does not retrieve ignore patterns
from relevant variables correctly. Now the ignored values are included
in the actual command.
Merge pull request #522 from tuhdo/helm-projectile-search

Make helm-projectile-grep ignores files properly and allow helm-projectile-ag to ignore files/directories